The fascinating HBO drama “The Leftovers,” whose first season concludes tonight, is a thought experiment of the greatest intrigue: what if 2 percent of the world’s population disappeared without trace or explanation?

Much of the shell of society, its political structures and bureaucracies, survives in this alternate world. Hollow political gestures persist. The anniversary of the Sudden Departure, a Rapture-like event that is too random a harvest to be the actual Rapture, is named “Heroes Day” and marked with a parade and wallowing speeches. The mayor of Mapleton, the fictional town where the show is set, is frank about the name: she knows that many of the disappeared were far from heroic, but reasons that no one will turn out for “We Don’t Know What Happened Day.” (The mayor won her office after what political scientists might see as the ultimate election campaign surprise, we learn. “Barring a miracle, I don’t think you stand a chance” campaign advisor tells candidate, a day before the Sudden Departure).

What would society look like if our beliefs about the world became instantly untenable? The random mass disappearance destroys the core assumptions of science and religion, so the people of this alternate reality sink into nihilism. This is exemplified by the cult group the Guilty Remnant, whose members wear shapeless white smocks and chain-smoke on the principle that the new world will not last long enough for any of them to get cancer.

The show has a post-9/11 sensibility. A new federal agency, the Department of Sudden Departure, busies itself with administering lengthy questionnaires to the loved ones of the disappeared. As with the “security theater” of our Transportation Security Administration, which some criticize as being more about showy reassurance than safety, those left behind suspect that the questionnaires are tossed aside without analysis, as the government has privately given up hope of understanding the new world but can’t say so in public.

In most post-apocalypse thought experiments the majority is wiped out and a small remainder fight for continued survival. Here, the majority is left behind but their will to carry on is crushed by mass grief and the lack of an explanation for what has happened. “The Leftovers” is even more grueling than “Battlestar Galactica,” which had seemed to push the outer limits of how unremitting a post-apocalypse TV show could be, and is perhaps most similar to the movie “Children of Men,” about an aging society seeing out the shabby remnants of its life after the population becomes infertile.

The most vivid character, one of the few who seems not to have surrendered to a smothering depression (although it is an open question whether he has retained his grip on reality), is the police chief Kevin Garvey, played by series star Justin Theroux. Garvey still cares enough to get angry over turf wars with the federal government, and to react with humanity when the reconfigured Department of Alcohol, Tobacco, Firearms, Explosives and Cults (ATFEC) offers to “eliminate the infestation” of a Guilty Remnant chapter in his town. In one of the very few instances of humor in the entire season, Garvey toasts a bagel which fails to emerge from the toaster oven – has his breakfast been raptured? It seemed that this might be an on-going gag when all of Garvey’s white shirts also disappear, yet even this small trace of levity quickly turns ominous.

“The Leftovers” is a collaboration between Tom Perrotta, from whose novel the series is adapted (the politically minded might know him from the book and movie “Election”), and Damon Lindelof, who is best known for the TV series “Lost.” Perrotta’s exceptional novel beautifully imagined the post-Departure world, and his characters have made it on-screen with just a few televisual tweaks (HBO’s Garvey family is TV beautiful rather than a bit oddball as portrayed on the page, and Kevin Garvey is a humdrum local politician in the book rather than the tortured police chief of the show). We get a little more political background in the novel, as Perrotta tells us that the post-Departure society has lost interest in enforcing most of its rules and in expending angst on future catastrophes like global warming. Those issues that seemed so urgent in a world amenable to rational analysis look absurd following the Sudden Departure.

The show’s forward plot momentum comes from Lindelof, who has scattered puzzles and portents throughout the first season. Lindelof did this with some panache on “Lost” too, before delivering a payoff that the audience didn’t like, and some have resisted investing this time as they lack faith that the mysteries will be explained. Yet the inexplicability of the Sudden Departure is the most interesting aspect of “The Leftovers.” Perrotta’s novel never explained what happened to the disappeared (although it did succumb to the perceived need to end dark tales on a note of hope), and the questions posed by “The Leftovers” are more engaging than any trite answers the show might give us in Sunday’s season finale and beyond.

Stephen Benedict Dyson is an associate professor of political science at the University of Connecticut. Follow him on twitter @sbdyson.